Gas sensor

ABSTRACT

Gas sensor materials composed of carbon mixture or metal-containing carbon mixture obtained as evaporated matter by arc discharge which occurs by passing an alternating current or a direct current with electric current density of 0.8 to 3.5 A/mm 2  on discharge surfaces of carbon electrodes or metal-containing carbon electrodes in an inert gas under a pressure of 0.1 to 600 torr.

TECHNICAL FIELD

The present invention relates to gas sensor materials.

BACKGROUND ARTS

A gas sensor responds to a specific gas contained in other gases such asair and produces an electric signal, a light signal and the likedepending on the concentration of the specific gas. Currently, variousgas sensors using a detection method wherein chemical properties such asadsorption, reaction, and light emission are often used to identify gascomponents have been known. As a conversion method into a signal, thereis an energy-conversion method, for example using electromotive force,which directly obtains a sensing signal by contacting gases. However, anenergy-control method which converts a change in device properties suchas material properties, e.g., electrical resistance, transistorproperties and the like, into a signal, is mainly used.

As typical materials for gas sensors, oxides having rather difficultreductiveness, such as SnO₂ and ZnO types, are available. Among them,most current commercial devices use a porous sintered body of the SnO₂type, which is n type semiconductor. As semiconductor gas sensors whichuse such an oxide semiconductor, there are surface control types inwhich interactive reaction with a gas stays on the surface and bulkcontrol types in which the reaction extends to a semiconductor itself.Among them, most are the surface control type like sensors ofcombustible gases- In such surface control type sensors, a chemicalreaction occurs on a semiconductor surface. However, the activity isoften not enough with a pure semiconductor. Therefore, the sensingfunction is improved by dispersing powder of a noble metal, a metaloxide and the like on the semiconductor particles.

However, to detect a gas by using these conventional gas sensors, it isrequired to heat the gas up to a temperature on the order of 200° C. to300° C. Moreover, with those sensors there is insufficient selectivityof gas species because of difficulty in specifying gas species sincethese conventional gas sensors may respond to many species of gases.Therefore, further improvement is desired.

The main object of the present invention is to provide sensor materialswhich are able to sense gases at room temperature, and which also aresuperior in selectivity of the gases sensed.

DISCLOSURE OF THE INVENTION

As a result of a series of research efforts related to the above object,the inventors found that a carbon mixture containing fullerenes obtainedby a carbon electrode arc method shows peculiar properties of adsorbinga polar gas and not adsorbing a nonpolar gas at room temperature, haslarger surface area and electric conductivity so as to be electricallymeasured because it is composed of carbon grains. Based on thesefindings, the inventors evaluated the usefulness of the carbon mixtureas a gas sensor material and found that the mixture can sense a gas at aroom temperature and is superior in gas selectivity. Further, such acarbon mixture that contains metal obtained by arc discharge, wherein acarbon electrode arranged so as to contain a metal component is used, isimproved in sensing ability of gases and has good selectivity, resultingin the present invention.

Namely, the present invention relates to providing the following gassensor materials:

(1) gas sensor materials composed of carbon mixture obtained asevaporated matter by arc discharge which occurs by passing analternating current or a direct current with electric current density0.8 to 3.5 A/mm² on discharge surfaces of carbon electrodes in an inertgas under a pressure of 0.1 to 600 torr.

(2) gas sensor materials composed of metal-containing carbon mixtureobtained as evaporated matter by arc discharge which occurs by passingan alternating current or a direct current with electric current densityof 0.8 to 3.5 A/mm² on discharge surfaces of carbon electrodescontaining metal at 0.01 to 30% by weight in an inert gas under apressure of 0.1 to 600 torr.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an arc discharge apparatusused in EXAMPLE 1;

FIG. 2 is a graph showing test results for sensing an ammonia gas usingthe carbon mixture obtained in EXAMPLE 1;

FIG. 3 is a graph showing test results for sensing an ammonia gas usingthe Ni-containing carbon mixture obtained in EXAMPLE 2;

FIG. 4 is a graph showing test results for sensing triethylamine usingthe Ni-containing carbon mixture obtained in EXAMPLE 2;

FIG. 5 is a graph showing test results for sensing a nitrogen monoxidegas using the Ni-containing carbon mixture obtained in EXAMPLE 2; and

FIG. 6 is a graph showing test results for sensing a nitrogentrifluoride gas using the Ni-containing carbon mixture obtained inEXAMPLE 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The carbon mixture as a gas sensor material of the present invention isobtained by arc discharge using carbon electrodes in an inert gas.

Conventional vacuum devices for arc discharge may be used as anapparatus for producing the carbon mixture.

As inert gases, for example, helium, argon, neon, krypton, xenon, andthe like may be used. The pressure of the inert gas may be about 0.1 to600 torr, preferably about 10 to 400 torr.

As electrodes for the arc discharge in producing the carbon mixture,carbon electrodes are used for both a cathode and an anode.

The arc discharge may be conducted by passing an alternating current ora direct current, wherein the electric current density on the dischargesurfaces of the carbon electrodes, or the electric current density onopposite surfaces of a cathode and an anode facing each other and spacedat a specified distance is about 0.8 to 3.5 A/mm².

The carbon mixtures obtained by such a method may be used for the gassensor materials of the present invention. The carbon mixture isso-called carbon soot, which can be obtained as powder attached to theinside of the apparatus. The carbon mixture comprises ultrafine carbongrains having a size of about φ1 nm to 100 μm, which are mainly composedof graphite carbon, indeterminately-formed carbon, and fullerenes,wherein the fullerenes are contained at about 0.1 to 15% by weight.

In the present invention, such gas sensor materials that contain metalcomponents in the above carbon mixture may also be used. The sensingability can be improved by adding such a metal component into the carbonmixture. Further, the gas sensor shows a peculiar property in accordancewith the metal species contained therein and is improved in selectivityagainst gases. For example, in the case of using a nickel-containingcarbon mixture, output form in direct current becomes very peculiarunder an atmosphere of nitrogen trifluoride, so that the identificationof the gas species becomes easier.

As the metal components of the metal-containing carbon mixture for thesubject sensor materials, metals of a typical element and transitionmetals may be used. For example, B, Mg, Al, Si, In, and the like may belisted as the metals of a typical element while La, Ni, Co, Fe, Cr, Ta,Mn, Mo, Ti, Au, Pd, Pt, Ag, and the like may be listed as the transitionmetals. These metal components may be used alone or in combination oftwo or more, wherein a suitable content is about 0.01 to 30% by weight.

The metal-containing carbon mixture can be obtained by arc discharge inan inert gas using the metal-containing carbon electrodes in the samemanner as obtaining the carbon mixture. The shape and the like of themetal-containing carbon electrodes may not be specifically limited andmay be the same as those electrodes conventionally used for arcdischarge, while the metal content in the electrode may be arranged insuch a manner that the metal content of the electrodes disappearing asevaporation by arc discharge may be the same as that of the carbonmixture produced by the arc discharge. As to methods to provide theelectrode with metal, there are a method wherein carbon and metallicpowder are homogeneously mixed so as to be formed into an electrode, amethod wherein metal in a form of powder, line, block, or the like isfilled into a cavity made in an electrode, or the like, either whichwill do. The conditions for the arc discharge to produce a metalcontaining mixture may be the same as those for the carbon mixtureabove.

The resultant matter obtained by the arc discharge using themetal-containing carbon electrodes is a mixture of ultrafine carbongrains having a size of the order of 1 nm to 100 μm and metallicultrafine grains having the same size as the above carbon mixture. Theultrafine carbon grains are composed of graphite carbon,indeterminately-formed carbon, and fullerenes, wherein the fullerenesare generally contained at about 0.1 to 15% by weight in the ultrafinecarbon grains. The metal content in the metal-containing carbon mixtureis the same as that of the electrode, which disappears as evaporation bythe arc discharge.

According to the present invention, a gas sensor can be obtained byusing the carbon mixture or the metal-containing carbon mixture as asensor material obtained by the above methods. The shape of the gassensor may not be specifically limited and may be the same as that ofvarious conventional gas sensors. For example, the carbon mixture or themetal-containing carbon mixture may be formed into a pellet or a sphereor the like, the carbon mixture or the metal-containing carbon mixturemay be formed into paste so as to be printed on a ceramic substrate, ormetal of MOS transistor may be replaced with the carbon mixture or themetal-containing carbon mixture so as to be formed into a FET type. Themethods for forming a compact may not be specifically limited. Forexample, the carbon mixture or the metal-containing carbon mixture maybe pressed at high pressure. Alternatively, the carbon mixture or themetal-containing carbon mixture may be mixed with a binder such ashydrocarbons (e.g., paraffin), polyvinyl acetate plastics, polyvinylalcohol, carboxymethyl cellulose, oil pitch, coal pitch, which arecontained at the order of 5 to 10% by weight based on the whole amount,dispersed in media such as water, alcohols, methyl cellosolve, or thelike. This mixture then may be formed into a desired shape, solidifiedand dried, and heated at about 200° C. to 300° C., and then may be bakedat about 400° C. to 900° C., if necessary, thereby resulting in acompact. Alternatively, colloidal silica may be added at about 1 to 20%by weight into the carbon mixture or the metal-containing carbonmixture, and then formed and dried at a temperature from roomtemperature to 200° C. A metal alkoxide solution (e.g., a solutioncontaining 25 g of Si(OC₂ H₅)₄, 37.6 g of C₂ H₅ OH, 23.5 g of H₂ O, and0.3 g of HCl) may be added at about 1 to 30% by weight in the carbonmixture or the metal-containing carbon mixture and dried at atemperature from room temperature to about 200° C. so as to form acompact.

Since output voltage or current varies in accordance with gasconcentration in these gas sensors, it is possible to detect the gas. Inaddition, it is possible to detect the change in frequency of a hydrogenoscillator with a change in weight by fixing the carbon mixture or themetal-containing carbon mixture to the hydrogen oscillator so as toadsorb the gas.

According to a gas sensor using the sensor materials of the presentinvention, it is possible to detect a gas at a room temperature.However, since the sensitivity may be inferior according to the gasspecies, the sensor may be heated to about 50° C. to 200° C. to improvethe sensitivity. As a method of heating, a gas may continuously beheated to a fixed temperature. Alternatively, since the sensor materialof the present invention has properties that it adsorbs a gas atordinary temperatures while it emits the adsorbed gas when heated, a gascan be detected as compulsorily exhausting the adsorbed gas with pulseheating.

The gas sensor materials of the present invention have peculiarproperties that they respond to a polar gas, while they scarcely respondto a nonpolar gas such as methane, butane, and isobutane. Since theoutput form of current or voltage differs according to the kind of polargas, it is easy to identify the gas species by using this property.Namely, the sensor materials are superior in selectivity. As examples ofpolar gases which can be detected by using the sensor materials of thepresent invention, ammonia or a gas containing an amino group, aminessuch as methyl amine, sulfur compounds such as methyl mercaptan, or agas containing a thiol group, oxides such as NOx and Sox, or a gascontaining oxygen atoms, CFs such as methane trifluoride bromide anddichloromethane difluoride, or nitrogen trifluoride (NF₃), or a gascontaining fluorine atoms, chlorinated compounds such as dichlorosilane,trichlorosilane, or the like, or a gas containing chlorine atoms,hydrogenated compounds, such as arsine (AsH₃), phosphine (PH₃), or a gascontaining hydrogen atoms, or the like, may be listed.

EMBODIMENT

The present invention will now be further described with reference tothe Examples.

EXAMPLE 1

A carbon mixture was produced by using an arc discharge apparatus havingcarbon electrodes as shown in FIG. 1. In the apparatus, carbonelectrodes having φ20 mm and 500 mm length were used as anode 1 andcathode 2. After evacuating the inside of a vacuum tank 3, helium gaswas introduced into the vacuum tank 3 so that the pressure was 100 torr.Then, a direct current was passed to carbon electrodes 1 and 2 by usinga DC power source 4 for arc discharge so that the current density on thedischarge faces (opposite faces on each of the electrodes) become about2 A/mm², and the carbon mixture was evaporated. Thereafter, a nitrogengas for industrial use was introduced into the vacuum tank 3, so thatthe pressure was at atmospheric pressure and the carbon mixture attachedto the inside of the vacuum tank 3 was taken out.

0.2 g of the thus obtained carbon mixture was put into a die of φ14 mm,was loaded with load of 1 ton by a presser, and was formed into apellet.

A test for detecting an ammonia gas by using the thus obtained pellet ata room temperature by the following four-terminal method then wasconducted.

An acrylic container with a gas stirring fan having a capacity of 5,400ml was prepared. The above pellet made of the carbon mixture was placedon the inside bottom of the acrylic container. Four terminals arealigned in line with 2 mm intervals on the surface of the pellet and0.09 mA DC was passed to the two end terminals. The output voltage wasmeasured on the internal two terminals. After the output became steady,5 ppm of ammonia was injected into the container by a micro syringe, atime-course change of the output current was obtained and its resultsare shown in FIG. 2. From these results, it is found that the abovecarbon mixture shows good responsiveness to an ammonia gas at roomtemperature.

EXAMPLE 2

A cylindrical hole of φ3.2 mm with 15 mm depth was made on a centralpart of a bottom of a cylindrical carbon electrode of φ20 mm and 500 mmlength and the hole filled wits. 0.9 g of nickel powder (of 200 mesh) sothat nickel-containing electrodes were produced. Thus obtainedelectrodes were used as anode 1 and cathode 2. The surfaces on whichnickel was filled were placed so as to face each other at a fixedinterval for electric discharge. Using the same arc discharge apparatushaving carbon electrodes as in EXAMPLE 1, arc discharge was conducted atthe same conditions as those of EXAMPLE 1, so that the electrodes wereevaporated. As result, a nickel-containing carbon mixture was obtainedas deposited on the inside of the vacuum tank. The thus obtained carbonmixture contained 2.95% by weight of nickel with a particle size ofabout 200 Å.

By using the thus obtained nickel-containing carbon mixture as a gassensor material, a test for detecting a gas was conducted.

The same apparatus as that of EXAMPLE 1 was used and thenickel-containing carbon mixture was formed into a pellet. Fourterminals were provided in the same way as EXAMPLE 1 and 0.1 mA currentwas passed to the two end terminals. After 5 ppm of an ammonia gas wasinjected into the container by a micro syringe, a change in DC outputwas obtained when the lid of the container was opened after a specifictime has passed. The results are shown in FIG. 3.

Four terminals were provided in the same way as EXAMPLE 1 and 0.1 mAcurrent was passed to the two end terminals. After 10 μl oftriethylamine was injected into the container by a micro syringe, achange in DC output was obtained. The results are shown in FIG. 4.

Further, four terminals were provided on the pellet made ofnickel-containing carbon mixture by using the same test apparatus and0.1 mA current was passed to the two end terminals. After 8 ppm of anitrogen monoxide gas or a nitrogen trifluoride gas was injected intothe container by a micro syringe, a change in DC output was obtained.The results for a nitrogen monoxide gas are shown in FIG. 5 while thosefor a nitrogen trifluoride gas are in FIG. 6.

As apparent from the above results, the nickel-containing carbon mixtureshows superiority in sensitivity to various polar gases and alsoselectivity of gas species because it shows each peculiar output form inaccordance with the difference of gas species. Particularly, it is easyto identify an nitrogen trifluoride gas, since the output current formof the gas is special and different from other gases.

INDUSTRIAL APPLICABILITY

The gas sensor material of the present invention uses the carbon mixtureor metal-containing carbon mixture obtained by arc discharge as it is,that is, without purification. Therefore, since no purification and thelike is necessary, there is no trouble in generating wastes accompanyingto a purification and any cost increase.

Further, since the gas sensor materials enable gas detection at roomtemperature and shows superiority in gas selectivity, the materials arevery useful.

What is claimed is:
 1. A gas sensor comprising a main body formed ofcarbon mixture and terminals provided on the main body at apredetermined interval for measuring an output voltage or an outputcurrent between terminals, the carbon mixture having been obtained asevaporated matter by arc discharge generated by passing an alternatingcurrent or a direct current with electric current density of 0.8 to 3.5A/mm² on discharge surfaces of carbon electrodes in an inert gas under apressure of 0.1 to 600 torr.
 2. A gas sensor comprising a main bodyformed of metal-containing carbon mixture and terminals provided on themain body at a predetermined interval for measuring an output voltage oran output current between the terminals the metal-containing carbonmixture have been obtained as evaporated matter by arc dischargegenerated by passing an alternating current or a direct current withelectric current density of 0.8 to 3.5 A/mm² on discharge surfaces ofcarbon electrodes containing 0.01 to 30% by weight of metal in an inertgas under a pressure of 0.1 to 600 torr.